Test as you go
Don’t wait to the end
of model development to start testing.
Test as you build. If you wait
until the end to verify the model works correctly, it will take much longer to
find bugs because you have to search through the entire model. Build the
model in small chunks and test thoroughly as you go. If you test as you go, the new bugs will be
in the constructs you have just built.
You will be focused on a smaller construct to look for the bug while
that logic is fresh on your mind. This
is such a crucial aspect of the modeling process that it deserves mentioning
more than once!
Snipping Tool or Snagit or Print Screen
Get access to a
screen capture tool and keep it handy.
When an error message pops up, take a screenshot of it. Don’t be click happy and absent-mindedly
click the OK button when errors appear.
If you don’t document what the errors are, you will have to rerun the
model again because you will quickly forget the exact details of the error
message. I think the Snipping Tool is
installed with Windows, but I am a big fan of SnagIt.
Logging exceptions
No one can or should
constantly look for every potential bad event each time they run the
model. For example, if an item stays in
the system 10x longer than you expect, there might be a potential problem. Or if item type x ever goes through a
specific construct then you know you have a potential problem. You can and should continue to monitor that
event in case another bug shows up.
There are many more instances that require monitoring, but they
shouldn’t consume our attention.
These are events
where you need to set up a construct to log the “exception” events to an error
log table in the database when they occur.
You should also report the number of these exceptions on the main model
worksheet when and if they occur, so they are sure to grab your attention. Once you have finished verifying and
validating your model, then you can take the exceptions out or have a flag that
turns them off. You may also want to leave them in just to reduce your
worry. Logging error events or exception
events is a great way to monitor the system.
Note, however, that if you put error events in that are really warnings,
you might overlook real errors when they occur.
Find dialog
If you have an error
message and it refers to an object number, you can use the menu shortcut
"Control + F" to open the Find dialog in order to find the object
number.
Global Random Seed
If you need to debug
your model and want a repeatable result from run to run, you can fix the Global
Random Seed. Debugging can be difficult
if the bug appears at different simulation times from run to run. Fixing the global random seed can be done in
the Random Number tab of the Simulation Setup dialog. If you want to repeat the last model run, set
the Random Seed to the "Starting seed used in the last model run." Setting the seed to any positive number will
allow the model to run with a repeatable stream of random numbers, thus giving
repeatable results. Setting the seed to
0 or blank provides a random starting seed.
Releasing resources
Be careful when you release
resources. The Resource Pool Release
block is used to return the resource back to the appropriate pool when the
resource is no longer required. If the
block immediately downstream from the Resource Pool Release block has a capacity
limit and is full, the resource won’t get released when it is finished with the
upstream task. The resource won’t get
released until the item with the resource can move past the Resource Pool
Release block and into the next residence block downstream. This issue is easy to overlook. It will prevent the resource from being used
elsewhere even though it is done with its current task. The Resource Pool will overly estimate its
utilization. I suggest you specifically
look for this issue at the end of model development on every model. The following construct is one way to
eliminate the issue. In this construct,
when the Activity for Task X is busy, the item will be in the Activity labeled
"Task X Busy". When the Activity
for Task X is blocked, it will wait in the queue labeled "Task X
Blocked" while the Gate prevents anything new from arriving into the
Activity block.
Debugging infinite loops where the simulation time never
progresses past a certain point
Occasionally your
model might have a bug which causes it to go into an infinite loop. This loop could be due to several
reasons. The reasons could involve
either creating an infinite number of items, an item looping continuously in a
circle, from an unending for loop, or from several other reasons. There is an easy way to figure out where the
loop is occurring. Turn on the profiler
and run the model again well into the infinite loop. After a while, you must manually stop the
model but it should show you the block (S) where the excessive time has been
spent. The blocks with excessive times
should be where the infinite loop is occurring.
No comments:
Post a Comment